Gas turbines are known to comprise the following elements: a compressor for compressing air; a combustor for producing a hot gas by burning fuel in the presence of the compressed air produced by the compressor; and a turbine for expanding the hot gas produced by the combustor. Gas turbines are known to emit undesirable oxides of nitrogen (NO.sub.x) and carbon monoxide (CO). One factor known to affect NO.sub.x emission is combustion temperature. The amount of NO.sub.x emitted is reduced as the combustion temperature is lowered. However, higher combustion temperatures are desirable to obtain higher efficiency and CO oxidation.
Two-stage combustion systems have been developed that provide efficient combustion and reduced NO.sub.x emissions. In a two-stage combustion system, diffusion combustion is performed at the first stage for obtaining ignition and flame stability. Premixed combustion is performed at the second stage to reduce NO.sub.x emissions.
The first stage, referred to hereinafter as the "pilot" stage, is normally a diffusion-type burner and is, therefore, a significant contributor of NO.sub.x emissions even though the percentage of fuel supplied to the pilot is comparatively quite small (often less than 10% of the total fuel supplied to the combustor). The pilot flame has thus been known to limit the amount of NO.sub.x reduction that could be achieved with this type of combustor.
Pending U.S. patent application Ser. No. 08/759,395, assigned to the same assignee hereunder (the `395 application), is incorporated herein by reference and discloses a typical prior art gas turbine combustor 100. As shown in FIG. 1 herein, combustor 100 comprises a nozzle housing 6 having a nozzle housing base 5. A diffusion fuel pilot nozzle 1, having a pilot fuel injection port 4, extends through nozzle housing 6 and is attached to nozzle housing base 5. Main fuel nozzles 2, each having at least one main fuel injection port 3, extend substantially parallel to pilot nozzle 1 through nozzle housing 6 and are attached to nozzle housing base 5. Fuel inlets 16 provide fuel 102 to main fuel nozzles 2. A main combustion zone 9 is formed within a liner 19. A pilot cone 20, having a diverged end 22, projects from the vicinity of pilot fuel injection port 4 of pilot nozzle 1. Diverged end 22 is downstream of main fuel swirlers 8. A pilot flame zone 23 is formed within pilot cone 20 adjacent to main combustion zone 9.
Compressed air 101 from compressor 50 flows between support ribs 7 through main fuel swirlers 8. Each main fuel swirler 8 is substantially parallel to pilot nozzle 1 and adjacent to main combustion zone 9. Within each main fuel swirler 8, a plurality of swirler vanes 80 generate air turbulence upstream of main fuel injection ports 3 to mix compressed air 101 with fuel 102 to form a fuel/air mixture 103. Fuel/air mixture 103 is carried into main combustion zone 9 where it combusts. Compressed air 12 enters pilot flame zone 23 through a set of stationary turning vanes 10 located inside pilot swirler 11. Compressed air 12 mixes with pilot fuel 30 within pilot cone 20 and is carried into pilot flame zone 23 where it combusts.
FIG. 2 shows a detailed view of a prior art fuel swirler 8. As shown in FIG. 2, fuel swirler 8 is substantially cylindrical in shape, having a flared end 81 and a tapered end 82. A plurality of swirler vanes 80 are disposed circumferentially around the inner perimeter 83 of fuel swirler 8 proximate flared end 81. Fuel swirler 8 surrounds main fuel nozzle 2 proximate main fuel injection ports 3. Fuel swirler 8 is positioned with swirler vanes 80 upstream of main fuel injection ports 3 and tapered end 82 adjacent to main combustion zone 9. Flared end 81 is adapted to receive compressed air 101 and channel it into fuel swirler 8. Tapered end 82 is adapted to fit into sleeve 86. Swirler vanes 80 are attached to a hub 87. Hub 87 surrounds main fuel nozzle 2. Fuel swirler 8 is attached to liner 19 via attachments 89 and swirler base 99.
FIG. 3 shows an upstream view of combustor 100. As shown in FIG. 3, pilot nozzle 1 is surrounded by pilot swirler 11. Pilot swirler 11 has a plurality of stationary turning vanes 10. Pilot nozzle 1 is surrounded by a plurality of main fuel nozzles 2. A main fuel swirler 8 surrounds each main fuel nozzle 2. Each main fuel swirler 8 has a plurality of swirler vanes 80. The diverged end 22 of pilot cone 20 forms an annulus 18 with liner 19. Main fuel swirlers 8 are upstream of diverged end 22. Fuel/air mixture 103 flows through annulus 18 (out of the page) into main combustion zone 9 (not shown in FIG. 3).
It is known that gas turbine combustors such as those described in FIG. 1 emit oxides of nitrogen (NO.sub.x), carbon monoxide (CO), and other airborne pollutants. While gas turbine combustors such as the combustor disclosed in the `395 application have been developed to reduce these emissions, current environmental concerns demand even greater reductions.
It is known that leaner fuel/air mixtures burn cooler and thus decrease NO.sub.x emissions. One known technique for providing a leaner fuel mixture is to generate turbulence to homogenize the air and fuel as much as possible before combustion to eliminate rich zones which would result in localized hot regions ("hot spots").
Fuel swirlers having swirler vanes such as those described above have been used to generate premix turbulence to create lean fuel/air mixtures. The swirler vanes create an obstruction in the path of the compressed air as it moves through the fuel swirler. This obstruction causes a pressure drop within the fuel swirler. Since the pressure of the fuel/air mixture moving into the main combustion zone directly affects the air-to-fuel ratio (AFR) in the main combustion zone (by affecting the intra-combustor air distribution), a higher pressure drop within the fuel swirler reduces the AFR. While turbulence is necessary to premix fuel and air, if too much turbulence is carried into the main combustion zone, recirculation zones are formed, increasing the risk of flame holding.
Thus, there is a need in the art for a fuel swirler that reduces NO.sub.x and CO emissions from gas turbine combustors by optimizing the amount of premix turbulence generated to provide more evenly distributed fuel/air mixtures without increasing the risk of flame holding or flashback.
Additionally, swirler vanes are generally of a fixed geometry and provide relatively little control over the pressure drop in the fuel swirler. Similarly, a set of swirler vanes which optimizes the AFR for one combustor generally will not be optimal for combustors of other sizes. The costs associated with varying the size of the swirler vanes to optimize pressure drop, or to accommodate different sized combustors is generally quite high.
Thus, there is a need in the art for a fuel swirler that reduces NO.sub.x and CO emissions from gas turbine combustors by providing greater control over the pressure drop within the fuel swirler, while increasing the flexibility and decreasing the costs associated with optimizing the AFR in combustors of different sizes.